1. Field of the Invention
The present invention relates generally to telecommunication systems. More particularly, the present invention is directed to a method for dynamically obtaining in real time the level of the power control signal at a base station
2. Description of the Related Art
Code Division Multiple Access (CDMA) is a form of modulation used in telecommunication systems. In CDMA, digital information is encoded in expanded bandwidth format and signals are transmitted simultaneously within the same bandwidth. Mutual interference between signals is reduced by spreading gain between unique codes used for each signal. CDMA permits a high degree of energy dispersion in the emitted bandwidth.
In CDMA systems, the number of signals which can be transmitted simultaneously is limited by the total power of the transmitted signals. Thus, reducing the power of the signals increases the capacity of the telecommunication system. However, reducing the power of a signal increases the error rate of that signal. To maintain minimum power for a given error rate, telecommunication systems employ power control loops.
A typical mobile cellular telecommunication system power control loop varies the power output of the mobile station to maintain a constant frame error rate at the base station. Frame error rate is the number of frame errors divided by the total number of frames observed. A frame error occurs when one or more bit errors occur in a frame of bits. Frame errors are detected after error correction. A frame error rate target is selected to minimize power without compromising signal quality. If the frame error rate exceeds the frame error rate target, the usefulness of the signal is reduced and the power output level of the mobile station is increased to decrease the number of frame errors. If the frame error rate is below the frame error rate target, the power output level, and the power output level of the mobile station is reduced.
A typical frame error rate target for a power control loop is 1%. To develop a confidence level in the frame error rate estimation and control, several frame errors must be observed. Because frame errors occur approximately once out of every 100 frames (assuming the frame error rate target is 1%) and several frame errors are required to develop a confidence factor, the power output level target for the mobile station may only be adjusted once every several hundred frames. During this several hundred frame period, the propagation losses between the mobile and the base station can vary due to movement of the mobile station and interference. This propagation loss variation causes a received power variation in the base station. To accommodate this variable power loss, the mobile station must increase its power output level so that the power loss variations do not decrease the power level at the base station below the minimum level required for the target error rate. As discussed above, the capacity of a CDMA system is determined by the total power of the transmitted signals. Thus, the increased power level to accommodate varying power loss between adjustments decreases the capacity of the telecommunication system.
In a CDMA wireless communication system, power control is used to maximize the capacity of the system. Subscriber units' output power must be controlled to guarantee enough signal strength received at the base station and to maintain good quality audio while minimizing the potential for interference. Since a CDMA wideband channel is reused in every cell, self interference caused by other users of the same cell and interference caused by users in other cells is the most limiting factor to the capacity of the system. Due to fading and other channel impairments, maximum capacity is achieved when the signal-to-noise ratio (SNR) for every user is, on the average, at the minimum point needed to support acceptable channel performance. Since noise spectral density is generated almost entirely by other user's interference, all signals must arrive at the CDMA receiver with the same average power. This is achieved by providing dynamic power control of the mobile station transceiver.
Power control of the mobile station's transmitter consists of two elements: open loop estimation of transmit power by the mobile station, and closed loop correction of the errors in this estimate by the base station. In open loop power control, each mobile station estimates the total received power on the assigned CDMA frequency channel. Based on this measurement and a correction supplied by the base station, the mobile station's transmitted power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level. All mobile stations use the same process and arrive with equal average power at the base station. However, uncontrolled differences in the forward and reverse channels, such as opposite fading that may occur due to the frequency difference and mismatches in the mobile station's receive and transmit chains, can not be estimated by the mobile.
To reduce these errors, each mobile station corrects its transmit power with closed loop power control information supplied by the base station via low rate data inserted into each Forward Traffic Channel. The base station derives the correction information by monitoring the Reverse CDMA Channel, compares this measurement to a threshold, and requests either an increase or decrease depending on the result. In this manner, the base station maintains each reverse channel, and thus all reverse channels, at the minimum received power needed to provide acceptable performance.
In a CDMA wireless communication system as described above, a predetermined number of radio frequency resources, such as transceivers and channel modulator/demodulators (modems) are located at each base station. The number of resources allocated to a particular base station is a function of the anticipated traffic loading conditions. For example, a system in a rural area may only have one omni-directional antenna at each base station, and enough channel modems to support eight simultaneous calls. On the other hand, a base station in a dense urban area may be co-located with other base stations, each having several highly directional antennas, and enough modems to handle forty or more simultaneous calls. It is in these more dense urban areas that cell site capacity is at a premium and must be monitored and managed closely in order to provide the most efficient allocation of limited resources while maintaining acceptable quality of communications.
Sector/cell loading is the ratio of the actual number of users in the sector to the maximum theoretical number that the sector can support. This ratio is proportional to total interference measured at the receiver of the sector/cell. The maximum number of users that the sector/cell can support is a function of the aggregate signal-to-noise ratio, voice activity, and interference from other cells. The individual subscriber unit signal-to-noise ratio depends on subscriber unit speed, radio frequency propagation environment, and the number of users in the system. Interference from other cells depends on the number of users in these cells, radio frequency propagation losses and the way users are distributed. Typical calculations of the capacity assumes equal signal-to-noise ratio for all users and nominal values of voice activity and interference from other cells. However, the signal-to-noise ratio changes from user to user and frequency reuse efficiency varies from sector to sector. Hence there is a need to continuously monitor the loading of a sector or cell.
A conventional way to monitor cell site loading conditions is for a person, usually a network engineer or technician employed by a wireless communication service provider, to travel from cell to cell making loading condition readings using specially designed and expensive test equipment. The logged data is then returned to a central processing facility for post-processing and analysis. Some significant drawbacks to this method are that the data can not be evaluated in real-time, and that significant errors are introduced due to propagation effects between the base station and the measurement equipment. Thus, this monitoring method only provides a rough estimate of cell site loading conditions, and can only be used in a time-delayed fashion to take corrective action, such as reassigning resources for the future. It does not enable the service provider to take any real-time action to improve loading conditions and their effect on system performance. Additionally, it requires a person to travel to each site serially, thus providing a discontinuous “hit or miss” estimate of the peak loading condition and consequent system performance depending on whether the visit coincided with the actual (rather than assumed) peak usage times.
Another possible way of monitoring cell site loading conditions is to access the performance data logged by the base station itself, or the base station controller. This procedure suffers from the non-real time post-processing problems as previously mentioned. What is needed is a simple and accurate real-time load monitoring system.